This invention relates to four-stroke spark-ignition engines in which a tumbling motion is imparted to the intake charge during its induction into the engine cylinder, and in preferred variations, it relates to such engines as adapted for stratified charge operation.
The pentroof style of combustion chamber is often designed to include two poppet type of intake valves located on one side of the pentroof in positions symmetric with respect to a central plane passing through the geometric axis of the associated engine cylinder. It has long been known that such a configuration facilitates the generation during the intake stroke of a vortex swirling about an axis perpendicular to the geometric axis of the cylinder. If the individual induction flow rates through the two intake valves are carefully balanced by means such as a continuation of the symmetry over the entire length of the two individual intake tracts, then the charge vortex will consist of siamese barrel swirl layers which also conform to the symmetry. As a result, the plane of symmetry will also be a boundary which defines flow separation between the two barrel swirl layers delivered through the two intake valves.
As the associated piston advances toward the pentroof chamber during its compression stroke, the two barrel swirl layers will experience radical alteration in shape, eventually becoming two flattened, but still symmetric and thus unmixed, flow-vortices with their now individual axes of rotation positioned nearly in parallel with the geometric axis of the engine cylinder. Mitsubishi engineers have verified that this breaking apart of the bulk charge motion into two mirror-image halves persists nearly until top dead center, where the charge motion is converted into large scale eddies. Not only does the verification of essentially no mixing across the plane of symmetry include computer modelling using computational fluid dynamics, but also empirical data obtained using a two-color laser sheet method as applied to a motored engine with optical access (described in SAE paper 920678). Most emphatically, this persistent separation of charge motion virtually guarantees a problem with air utilization at high BMEP (brake mean effective pressure) if fuel is always metered into only one of the two intake passages to thereby implement a fully barrel-stratified mode of engine operation.
While it is obvious that a less rudimentary fuel metering system can easily avoid the high-BMEP air utilization problem (excessive particulate matter, etc.) by allowing the two barrel swirl layers to have independently selected air-fuel ratio values; that is not the approach that has been taken either in barrel stratified engines mass produced for passenger cars, or apparently in any prototype engine that has actually been built and tested. What has resulted from this reluctance to include the necessary fuel metering capabilities is a failure to promote and preserve the stratification, most likely to the extent that the true potential of the barrel stratified charge engine remains obscure. In other words, design carelessness and even intentional degradation of the desirable in cylinder flow separation have typified previous research and development experience with barrel stratification. Such failures likely rule out successful implementation of features compulsory for barrel stratification to make spark ignition engine performance competitive with the fuel efficiency achieved by turbocharged Diesel engines which are drastically more costly to mass produce than barrel stratified engines with the requisite fuel metering capabilities.
My U.S. Pat. No. 6,612,285 discloses how the classic four-valve pentroof combustion chamber can be adapted to accommodate both barrel-stratified and homogeneous-charge modes of engine operation in order to significantly close the fuel efficiency gap separating Diesel engines from much less costly spark ignition engines. In more detail of this prior disclosure, the fully barrel stratified charge mode of engine operation becomes activated when fuel is in effect completely missing from a segregated half of the stoichiometric intake charge filling each engine cylinder at the conclusion of the intake stroke of the associated piston. Since the associated multi-cylinder engine would include features devised to promote and preserve the charge stratification to the full extent practical, consistent spark ignition can be achieved at wide open throttle in this very lean mode of engine operation. Not only that, but progressive addition of fuel to the originally unfueled barrel swirl layer (without increasing fuel metered to the originally fueled layer) will progressively increase BMEP in this wide open throttle regime until homogeneous charge operation is reached at a stoichiometric overall air-fuel ratio. To decrease BMEP while continuing to confine stoichiometric fueling to one barrel swirl layer will, in contrast, require use of the engine throttle valve. Well defined stratification at the time of ignition would assume even greater importance in this throttled regime of lower BMEP operation.
Another advantage conferred by really pronounced stratification of the two barrel swirl layers is that a spark plug originally in a central location need be offset only moderately into the always-fueled one of the two barrel swirl layers in order to provide excellent ignition performance in the barrel stratified mode of engine operation as well as in the homogeneous mode. However, the invention of my prior-art U.S. patent noted above employs two spark plugs per engine cylinder rather than just one moderately offset spark plug.
Like the present invention, some of the prototype barrel-stratified engines investigated by Ricardo Consulting Engineers utilized the simpler expedient of having just one spark plug in each engine cylinder. (Although the cost differential between turbodiesel and spark ignition engines would easily justify the more complex ignition system of my prior art invention, cost reduction always garners extra interest in the automotive industry.) Another feature of the present invention briefly considered in a basic way by Ricardo Consulting Engineers in their SAE paper 940482 is using intake-valve flow masks to generate tumble (the more common designation for barrel swirl, except when considering stratified charge engines) rather than geometric swirl. Nevertheless, Ricardo's flow mask configuration does not readily accommodate the moderately offset spark plug position favored by Ricardo's engineers as well as by the present invention.
One example in particular helps confirm that to appreciably narrow the fuel efficiency gap separating turbodiesel engines from much less costly port-injected engines could not have been a serious objective of any barrel stratified engine that has yet been tested. SAE paper 950165 documents that, in the development of a multi-cylinder research engine, a production passenger car engine was modified by using only a single fuel injector to serve just one of the two completely separate intake passages associated with each engine cylinder, and also by offsetting the single spark plug in each cylinder toward its fueled side. Also clearly documented is the fact that short and long intake passage lengths served each cylinder, with the specific purpose of reducing the degree of stratification as a way of providing air utilization acceptable for full-load engine operation. Just a few months prior to publication of this SAE paper, a different company introduced a spark-ignition V6 into high-volume automotive production with many of the same features used by the research engine, most notably a single fuel injector in just one of two unequal length intake passages per cylinder. The production engine, however, was not even considered to be a stratified charge engine and did not employ any offset of its spark plug position from the central location normal for the classic four-valve pentroof combustion chamber.